The present invention relates to a multilayer parison extruder and, in particular, to a multilayer parison extruder in which at least four inlet ports, for introducing a corresponding one of a plurality of auxiliary materials into a corresponding one of a plurality of cylindrical slots provided in a multilayer head, are located at nearly equiangular intervals along the circumference of the head so that the uniformity of the thickness of the circumferential wall of a multilayer parison formed by the extruder is enhanced.
In a conventional multilayer parison extruder disclosed in the Japanese Patent Application (OPI) No. 99115/87 (the term "OPI" as used herein means an "unexamined published application"), cylindrical slots for auxiliary materials are provided in a multilayer head disposed in a cylinder, and two inlet ports are provided for each of the cylindrical slots and opposed to each other in the diametral direction of the cylindrical slot. Each of the auxiliary materials is supplied into the corresponding cylindrical slot through the two inlet ports so that a cylindrical multilayer parison, in which a plurality of layers are made of the auxiliary materials between layers of a main material, is extruded. However, since each of the auxiliary materials is supplied into the corresponding cylindrical slot through the two inlet ports opposed to each other in the diametral direction of the slot, the thickness of the auxiliary material layer at the points along the diametral direction of the slot which are located perpendicular to the position of the inlet ports is made unequal with respect to the thickness of the portion of the layer which is located proximate to the inlet ports. For that reason, there is a problem in that the thickness of the parison is nonuniform so as to deteriorate the yield thereof.